An Ultra Dense Network (UDN) is typically ultra-densely deployed in highly populated areas such as hot spots, office building, or downtown area at cities, where there are demands of high data rate service. Currently, data traffic is boosting rapidly while there is a clear bandwidth limit in low frequency bands. Hence, it is necessary for UDN to utilize a higher carrier frequency and is a wider bandwidth in order to reach an even higher data rate. Accordingly, the UDN is supposed to operate over higher frequency, such as Millimeter-wave (mmW) frequencies ranged from 30 GHz to 300 GHz.
Since the UDN is expected to be deployed in high frequency bands where radio wavelengths are substantially smaller than those in conventional cellular networks being deployed in relatively low frequency bands, a considerable number of antenna elements can be implemented in communication nodes with small physical dimensions. For example, a typical number discussed is in the order of 64 antenna elements in an access point (AP). The large number of antenna elements will be used to employ beamforming technique to create beams with high directivity, which should help avoiding interference between links in the densely deployed networks.
However, since there are much more antennas for one communication node than what we have today, this kind of system has the hardware limitations from the point of view of maximizing performance. In particular, it would be desirable to have full control over each individual antenna element; however this would require a complete digital chain per antenna element, which is not feasible due to the high power consumption of the digital processing and Digital-to-Analog Conversion (DAC) at the large system bandwidths considered for the UDNs. In this case, the number of digital chains is always constrained to be an acceptable value less than the number of antennas so as to save the power consumption and implementation complexity.
In addition, in the UDN, one digital chain in an AP is used to dedicatedly serve one user equipment (UE) in downlink transmission. As such, the maximal number of UEs that can be supported simultaneously in a subframe is equal to the number of digital chains in the AP. Hence, the maximum number of UEs that can be served simultaneously in each subframe is constrained by the limited number of digital chains, which results in low transmission efficiency.